Displacement effects and scaling of ducted, supersonic flames

The paper describes a study of the mechanisms for generating pressure incre ase within a ducted, supersonic hydrogen-air flame. The combustor consisted of a constant-area rectangular duct with a centrally located fuel injector strut that spanned the width of the duct. The free stream flows, with tota l enthalpies of 5.6, 6.5 and 8.9 MJ/kg, were provided by a free-piston shoc k tunnel and the fuel was injected from a Ludwieg tube. The wall pressure i ncrease generated within the duct was successfully estimated by first reduc ing the effective core area of the duct by the combined displacement thickn ess of the mixing layers and wall boundary layers and then calculating the properties of the inviscid core flow assuming an isentropic compression. It was found that heat addition decreased the density of the mixing layer a nd subsequently increased its displacement thickness without greatly alteri ng the layer's velocity profile. If pressure gradients and shock waves have only a minor effect on turbulence production then the displacement thickne ss of the mixing and boundary layers on be assumed to be independent of the height of the duct. If the duct height is changed, the resulting pressure distribution within the new duct can be predicted using the displacement th ickness distribution from the original duct. The paper thus presents a form of scaling for supersonic combustion experiments. (C) 1998 by The Combusti on Institute.